The present invention relates to a gain-coupling distributed feedback type semiconductor laser device, more particularly relates to a gain-coupling distributed feedback type semiconductor laser device which emits light with a high light output power and enables an increase in the threshold current to be kept to a minimum.
A distributed feedback type semiconductor laser device comprises a predetermined layer structure of semiconductor materials and a cavity having a predetermined cavity length in which is formed a diffraction grating for periodically changing a real or imaginary part of a refractive index and feeding back only laser light of a specific wavelength so as to give wavelength selectivity.
A distributed feedback type semiconductor laser device of a type where only the real part of the refractive index changes periodically inside the cavity is called a refractive index-coupling type, while one of the type where both the real part and imaginary part of the refractive index change periodically is called a gain-coupling type or a complex-coupling type.
Note that in the present invention, the latter of the above types, that is, the type where at least the imaginary part of the refractive index changes periodically, is called a gain-coupling distributed feedback type semiconductor laser device.
Among these types of distributed feedback type semiconductor laser devices, the refractive index-coupling type generally oscillates in two modes near the Bragg wavelength. This is because the difference in the threshold gain between the two modes sandwiching the Bragg wavelength is small. Therefore, this type of laser device suffers from the problem of a difficulty in achieving a single mode oscillation operation. The yield in the single mode becomes lower.
On the other hand, in the case of a gain-coupling distributed feedback type semiconductor laser device, the difference in the threshold gain between the two modes at the two sides of the Bragg wavelength is large, so the yield in a single mode becomes high.
Some gain-coupling distributed feedback type semiconductor laser devices, however, form a diffraction grating inside the cavity by periodically arranging absorbing layers comprised of semiconductor materials absorbing the light of the oscillation wavelength of the device. This type is called an absorbing diffraction grating type.
In an absorbing diffraction grating type of gain-coupling distributed feedback type semiconductor laser device, by giving a loss to only one mode among the two modes at the two sides sandwiching the stop band, the difference in the threshold gain between the two modes is made larger during operation. Therefore, it is possible to realize a high single mode yield with this device. In this case, the peaks of the standing wave which the laser light of the oscillation wavelength forms in the cavity are positioned away from the periodically arranged absorbing layers. In other words, the standing wave which the diffraction grating in the cavity forms has a waveform with peaks avoiding the absorbing layers.
On the other hand, if considering the reflection at the end surfaces of the cavity, since the end surfaces have free end reflection, the peaks of the standing wave match with the positions of the end surfaces of the cavity.
Therefore, if the absorbing layers of the diffraction grating are positioned in a state not matching with the end surfaces of the cavity, the valleys of the standing wave formed by the laser light of the oscillation wavelength in the cavity and the valleys of the standing wave due to the end-surface reflection easily match, so this laser device can realize a low threshold current and a high emission efficiency.
The usual practice however is to form the end surfaces of the semiconductor laser device by cleavage. Since the positions of formation of the cleaved facets (cavity end surfaces) are randomly positioned with respect to the diffraction grating, however, various relative positions occur such as the position of the cleaved facets and the position of the absorbing layers of the diffraction grating matching or not matching. This means that the standing wave to be formed by the diffraction grating and the standing wave to be formed by the end-surface reflection will not necessarily match in all cases.
For example, when the positions of the cleaved facets match with the positions of the absorbing layers of the diffraction grating, the peaks and valleys formed by the diffraction grating become opposite in phase to the peaks and valleys of the standing wave formed by the end-surface reflection. As a result, the oscillation mode is affected by the loss due to the absorbing layers of the diffraction grating and, while single mode oscillation is achieved, an increase in the threshold current or a reduction in the emission efficiency ends up being induced.
On the other hand, when considering increasing the light output power of a distributed feedback type semiconductor laser device, in the past, the method has been adopted of making the reflectance of the front facet (emission end surface) low and making the reflectance of the rear facet for example at least a high 80% and increasing the ratio of the light output power from the front facet.
If the above method is applied so as to realize a higher light output power of a semiconductor laser device of the absorbing diffraction grating type, however, the following problem arises. That is, at the high reflectance rear facet, the valleys and peaks of the standing wave are emphasized by the end surface reflectance, so when these do not match the standing wave due to the distributed feedback, there is the problem that a large increase of the threshold current and a large reduction in the emission efficiency are caused.
An object of the present invention is to solve the problems occurring when trying to realize a high light output power from a gain-coupling distributed feedback type semiconductor laser device giving a low reflectance to the front facet and a high reflectance to the rear facet, in particular a gain-coupling distributed feedback type semiconductor laser device of the absorbing diffraction grating type, and to provide a gain-coupling distributed feedback type semiconductor laser device realizing a high ratio of light output power without causing an increase in the threshold current or a reduction in the emission efficiency.
To achieve the above object, according to the present invention, there is provided a gain-coupling distributed feedback type semiconductor laser device comprised of a diffraction grating at least at part of the inside of a cavity and having a gain or loss periodically changing,
a reflectance of one end surface (front facet) of the cavity being not more than 3% and the reflectance of the other end surface (rear facet) being larger than the reflectance of that one end surface and not more than 60%.
In particular, there are provided a gain-coupling distributed feedback type semiconductor laser device wherein the reflectance of the other end surface (rear facet) is 30 to 60%,
a gain-coupling distributed feedback type semiconductor laser device wherein said one end surface (front facet) and said other end surface (rear facet) are formed with at least one type of film selected from the group of SiO2, SiNx (0xe2x89xa6xxe2x89xa61.4), xcex1-Si, and Al2O3,
a gain-coupling distributed feedback type semiconductor laser device wherein the length of said cavity is at least 400 xcexcm, and
a gain-coupling distributed feedback type semiconductor laser device wherein said diffraction grating is an absorbing diffraction grating comprised of a semiconductor material absorbing oscillation wavelength light.